In an article published in the February 8, 2002, Science, a research team led by Howard Hughes Medical Institute investigator King-Wai Yau described the circuitry, which consists of a subset of nerve cells that carry visual signals from the eye to the brain. The scientists showed that circadian-pacemaker nerve cells almost certainly depend on a different light-sensing pigment, called melanopsin, than the conventional visual system, which relies on rod and cone photoreceptors arrayed across the retina.
Biological, or circadian, clocks operate on a 24-hour cycle that governs sleeping and waking, rest and activity, body temperature, cardiac output, oxygen consumption and endocrine gland secretion. In mammals, the internal circadian clock resides in the brain, and sunlight is the cue that resets this clock daily.
Basic insights into the circadian system could lead to improved treatment for such problems as jet lag and depression, and even help optimize drug treatments affected by the rhythmic changes in body hormones.
"It's been known for twenty years that the eyes are required to set the circadian clock, a process called photoentrainment," said Yau, who is at The Johns Hopkins University School of Medicine. "But over the last few years or so, increasing evidence has suggested that the retinal rods and cones are not the only receptors involved in light detection." For example, studies showed that mice genetically altered to lack functioning rods and cones still showed photoentrainment of their circadian clocks, said Yau. The same non-visual, light-sensing system also
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Contact: Jim Keeley
keeleyj@hhmi.org
301-215-8858
Howard Hughes Medical Institute
7-Feb-2002